1. Field of the Invention
Exemplary aspects of the present invention generally relate to an optical scanning device and an image forming apparatus such as a digital multi-functional system, and more particularly, to an image forming apparatus including the optical scanning device.
2. Description of the Background Art
With an image forming apparatus, for example a digital copier or the like, there is growing market demand for production of images of ever-higher quality at ever-higher speeds.
In order to achieve the necessary speed and quality, an optical scanning device using multiple beams is known to be effective. Such an optical scanning device requires adjustment of the beam pitch (scan line pitch) between the plurality of laser beams. Known pitch adjustment methods include a method of rotating a multi-beam light source unit about an optical axis and a method using a dedicated optical element to adjust the beam pitch.
However, a drawback to rotating the multi-beam light source unit about the optical axis is that reliable rotation of the light source unit becomes a critical factor. In order to achieve superior image quality, the pitch of each beam needs to be narrowed to provide the necessary density, thereby complicating adjustment.
By contrast, use of the dedicated optical element to adjust the beam pitch necessitates use of a highly-accurate optical element made of glass, resulting in a cost increase.
Moreover, as noted above, to achieve high imaging quality, it is necessary to reduce the diameter of the beam spot in order to increase the number of spots per unit area (resulting in greater image density). Yet even if the diameter of the beam spot is adjusted with precision initially, unintended shifting may occur over time due to fluctuations in ambient temperature, for example.
In order to adjust the beam pitch, a liquid crystal element that is driven by an electric signal is used. The liquid crystal element is advantageous in that it is compact (light weight), driven with low voltage, and generates no heat, noise, or vibration.
Generally, such a liquid crystal element includes a layer formed of a liquid crystal having a thickness ranging from a few micrometers [μm] to several tens of micrometers [μm] sandwiched between two glass substrates. The edges of the glass substrates are sealed with a sealing member. This configuration is known as a cell structure. With this configuration, when the surrounding temperature changes, in particular, when the temperature in the atmosphere increases, the liquid crystal layer undergoes thermal expansion, causing a center portion of the crystal element to swell and thus causing the glass substrate to deform. As a result, a lens effect (positive power) is induced. If such a liquid crystal element is employed in the optical scanning device or the like, the position of the beam waist changes undesirably, thereby degrading the precision of the beam spot diameter (the beam spot diameter increases).
Methods of fixing the liquid crystal element have been proposed. One example of a related-art fixing method of the liquid crystal element includes fixing the liquid crystal element to a holder/support member by applying an adhesive agent at a plurality of places along the periphery of the liquid crystal element.
A drawback to this approach is that changes in the temperature inside the image forming apparatus affect the amounts of thermal expansion of the liquid crystal element (that is, the glass substrate) and of the holder differently. Due to the difference in thermal expansion of the liquid crystal element and the holder, the liquid crystal element deforms, degrading optical characteristics such as wavefront aberration of the transmitted optical beam.
Furthermore, in the related-art approach, the liquid crystal element is adhered to a housing using the adhesive agent to protect the liquid crystal element from damage, and the related-art holder/support member does not include an opening for shaping the beam.
Furthermore, when the liquid crystal element is held by or fixed to the holder by pressing the sealed portion by a spring or the like, the glass substrate deforms, thereby causing localized degradation of parallelism of the thickness of the liquid crystal layer. As a result, the optical characteristics described above, such as wavefront aberration of the transmitted optical beam, are also degraded.